For heterojunction photonic devices, such as Ge/Si photodiodes or GaAs/Si photodiodes, some defect states are located around the interface. These defect states usually have an energy level of 0.06V above the valence band (acceptor) with a density range of 1011˜1013 cm−2. Because of the existence of such defect energy states, the region is more difficult to be depleted compared to intrinsic material, and it requires a higher applied bias for photodiode or avalanche photodiode operation. For high-speed device such as 25-GHz devices (25G) or beyond, such higher applied bias is undesirable and unacceptable. Moreover, high applied bias causes high electric field in this region, which will lead to high leakage current. Moreover, for high speed operations such as 25G or beyond, the absorption layer becomes very thin to ensure high bandwidth operations, thus the optical absorption becomes very low.